High voltage alternating current (AC) power capacitors are designed to meet the mechanical, electrical, and performance requirements of high voltage high frequency AC electrical circuits. Such capacitors commonly used in electrical circuits carrying peak voltages of, for example, 1400Vpeak and electrical current of 3000 Arms are prone to Ohmic, dielectric and inductive energy losses mainly in the form of heat. For example, in a common high and medium frequency (e.g., 1 kHz to 1 MHz) power capacitor each 500 kvar reactive power can generate a loss of 500 to 1000 Watt in the form of heat.
High voltage power capacitors are commonly multi-layered capacitors such as, for example, film capacitors that are made of alternating layers of a conducting material such as, for example, aluminum foil and a dielectric, such as, for example polypropylene film and are either layered into flat layers or rolled into a spool or a bobbin. Other dielectric materials can include polyester (Mylar®), polystyrene, polypropylene, polycarbonate, metalized paper, Teflon® and others. Electrodes are then either thermally bonded by, for example soldering or mechanically connected by, for example connectors to one or more edges of each of the external conductive layers on either side of the layered capacitor body or to each flat end of the bobbin, formed by edges of the conducting film windings.
The assembled capacitor is commonly potted thus providing isolation of the capacitor body or bobbin from the environment leaving only the electrodes exposed.
Though soldering provides a solid attachment between the electrodes and the capacitor body contact surface there are some disadvantages associated with soldering. Tin is commonly used as a soldering material and solder points act to transfer heat from the capacitor body (commonly aluminum-dielectric layers) to the electrode (commonly copper) and/or the environment by heat conduction and dissipation. However, in power capacitors, tin solder forms two interfaces: a capacitor body (commonly aluminum-dielectric layers)-tin interface and a tin-electrode (commonly copper) interface, which creates thermal junctions bringing about elevated junction temperature during capacitor operation.
The soldering tin, though a reasonable electrical conductor, still could have a resistance contributing to energy losses in such power capacitors.
There have been attempts to provide connections between two capacitor bodies arranged in series. U.S. Pat. No. 4,307,434 discloses a conductive sleeve or conductive tabs that are pressed between the layered capacitor conductive layers and “short circuit” two capacitors arranged in series.
U.S. Pat. No. 6,370,009 discloses replacing conventional soldering operations by crimping a wire to a foil edge of a capacitor.
Other solutions such as streaming cooling fluid through the capacitor housing or electrodes so that to drain heat away from the capacitor provide only a partial solution. In plastic films layered capacitors, the dielectric component/layers melt and/or disintegrate easily at temperatures of about 120 degrees Celsius whereas the soldering temperature may commonly reach over 200 degrees Celsius.
Soldering under water-cooling conditions alone allows for quick point soldering only (creating heat junctions) and does not enable soldering large areas between the capacitor body contact surface and the electrode since the high soldering temperatures generated by soldering large surface areas may damage the capacitor body by melting or disintegrating the dielectric component.
Additionally, since aluminum (capacitor body) cannot be directly soldered to copper (electrode) an intermediate bonding material must be added such as copper or tin-zinc powder that is arc sprayed over the surface of the capacitor body contact surface creating additional interfaces and thermal junctions.
The limitations of soldering as explained above also negate the option of assembling a double-body capacitor in which the capacitor bodies are arranged in series.